The reason that many quick breads, such as banana bread, sink in the middle after taking them out of the oven is due to several potential factors, such as being undercooked, not having enough leavening, allowing the batter to rest too long before baking, or having too high of a ratio of wet ingredients to dry. Banana bread is a dense quick bread, so a small amount of sinking can be expected, but not so much that a divot develops in the middle.
Even though this type of pastry is known as a quick bread, due to the lack of yeast in the ingredients, there are many things that can go wrong and ruin the bread. Overmixing the batter, for example, will lead to dry bread that looks as if it has numerous tunnels running through it. The wet and dry ingredients should be mixed separately, and then the dry ingredients should be added slowly by folding them in gently with a spatula. The batter may be lumpy, but will not affect the quality.
Other problems that can occur when making quick breads include bread that has a soap-like or bitter taste. This occurs when too much leavening has been added to the mixture. Too much sugar in the recipe will lead to bread that browns much faster than it should and could lead to a burnt surface. A combination of too much fat and leavening, which means baking soda, baking powder or both, will lead to a quick bread with a coarse texture. Ideally, quick breads like banana bread will be soft and tender when eaten.